1. Field of the Invention
The present invention relates to a technology used in a light source for exciting an optical fiber amplifier and capable of producing a laser beam of a stable wavelength.
2. Description of the Background
Conventionally, semiconductor laser devices producing laser beams as signal lights use a distribution feedback (DFB) type laser element incorporating a diffraction grating. Since a signal light is being produced, a wavelength of the signal light must be highly precise (e.g., xc2x10.1 nm or less). However, the wavelength of the signal light of such semiconductor laser devices are typically not stable. Accordingly, such laser devices require the use of a xe2x80x9cwavelength lock device.xe2x80x9d The wavelength lock device controls and monitors the wavelength of the laser beam produced by the laser device. The wavelength lock device controls the wavelength of the laser beam through feedback so that the monitored wavelength may be a desired wavelength, thereby producing a laser beam of a stable wavelength.
The wavelength lock device described above requires a large, complicated, and expensive optical system and control unit to monitor the wavelength of produced laser beam. Accordingly, it is difficult to reduce the size, weight, and cost of the semiconductor laser device.
Semiconductor laser devices producing laser beams as light sources for exciting Raman amplifiers also use a DFB type laser element incorporating a diffraction grating. However, the driving current applied to the semiconductor laser device for producing a laser beam as signal light is a small value (e.g., about 80 mA) as compared to the DFB laser element incorporating a diffraction grating used as a light source for exciting a Raman amplifier (e.g., the driving current increases about 1000 mA and a high output laser beam of over 300 mW is emitted). Along with the increase in the driving current of such Raman amplifier light sources, the temperature of an active layer of the laser device increases and the oscillation wavelength tends to shift to the longer wavelength side. Additionally, the semiconductor laser devices used as a light source for exciting a Raman amplifier require a wavelength control at a precision of about xc2x10.5 nm. The above factors render the use of conventional semiconductor laser devices used as a light source for exciting a Raman amplifier.
Generally, in the light source for exciting a Raman amplifier, a flat amplification characteristic is obtained by controlling the oscillation wavelength at a specific wavelength interval between plural semiconductor laser devices, but the amplification factor varies depending on whether the light input of the object of control is small or large, and hence it has been attempted to increase or decrease the light output of the Raman amplifier exciting light source depending on the magnitude of the light input. In other words, the driving current of the Raman amplifier exciting light source is increased or decreased in order to control the oscillation wavelength and thereby produce a flat amplification characteristic. As a result, as mentioned above, the temperature of the active layer changes and the oscillation wavelength shifts, and a flat amplification characteristic cannot be obtained.
The present invention provides a semiconductor laser device and a drive control method for a semiconductor laser device capable of obtaining a stable oscillation wavelength, in a simple, small and inexpensive structure, without requiring waveform monitor, regardless of a significant increase or decrease in driving current.
The present invention advantageously provides a semiconductor laser device including a semiconductor laser element, a temperature measuring element configured to measure a temperature, and a temperature regulating unit having the semiconductor laser element and the temperature measuring element thermally connected thereto. The semiconductor laser device also includes a current detecting unit configured to detect a driving current applied to the semiconductor laser element. Further, the semiconductor laser device includes a control unit configured to control the temperature regulating unit using a control function to achieve a substantially constant wavelength output from the semiconductor laser element. The control function defines a relationship between a predetermined driving current and a predetermined temperature. The control unit is configured to control the temperature regulating unit such that the temperature detected by the temperature measuring element substantially equals the predetermined temperature corresponding to the detected driving current as defined by the control function.
Additionally, the present invention advantageously provides a semiconductor laser device including a semiconductor laser element, a temperature measuring element configured to measure a temperature, and a temperature regulating unit having the semiconductor element and the temperature measuring element thermally connected thereto. The semiconductor laser device also includes a means for detecting a driving current applied to the semiconductor laser element. The semiconductor laser device further includes a means for controlling the temperature regulating unit using a control function to achieve a substantially constant wavelength output from the semiconductor laser element.
The present invention further advantageously provides a drive control method for a semiconductor laser device. The method includes the step of determining a control function defined as a relationship between a predetermined driving current and a predetermined temperature to achieve a substantially constant wavelength output from a semiconductor laser element of the semiconductor laser device. The method further includes a step of detecting a driving current applied to the semiconductor laser element. Further, the method includes controlling a temperature regulating unit such that a temperature detected by a temperature measuring element substantially equals the predetermined temperature corresponding to the detected driving current as defined by the control function. In this method, the semiconductor laser element and the temperature measuring element are thermally connected to the temperature regulating unit.
Additionally, the present invention advantageously provides a drive control method for a semiconductor laser device for controlling a temperature of a semiconductor laser element on a basis of a temperature detected by a temperature measuring element disposed near the semiconductor laser element thereby controlling a wavelength of a laser beam oscillated by the semiconductor laser element. The method includes the steps of detecting a driving current applied to the semiconductor laser element, and acquiring a relationship between driving current and temperature at which the wavelength of the laser beam oscillated by the semiconductor laser element is generally constant. The method further includes the step of controlling the temperature of the semiconductor laser element so that the detected temperature of the semiconductor laser element and the corresponding detected driving current satisfy the acquired relationship.
Furthermore, the present invention advantageously provides a drive control method for a semiconductor laser device for controlling a temperature of a semiconductor laser element on a basis of a temperature detected by a temperature measuring element disposed near the semiconductor laser element thereby controlling a wavelength of a laser beam oscillated by the semiconductor laser element. The method includes the steps of detecting a driving current applied to the semiconductor laser element, and acquiring a plurality of relationships between driving current and temperature in which the wavelength of the laser beam oscillated by the semiconductor laser element is generally constant. The method also includes the steps of setting a desired wavelength, and changing from a present relationship to a new relationship corresponding to the desired wavelength. Further, the method includes a step of controlling a temperature of the semiconductor laser element so that the detected temperature and the corresponding detected driving current satisfy the new relationship.